The water-gas shift (WGS) reaction is a chemical reaction in which carbon monoxide (CO) reacts with water vapor (H2O) to form carbon dioxide (CO2) and hydrogen (H2). The water-gas shift reaction is a highly significant industrial reaction, and is used in conjunction with reforming of methane and other hydrocarbons for the production of high purity hydrogen, among other applications.
The shift reaction is equilibrium limited and the extent of CO-conversion is dependent on the temperature in the WGS reactor utilized, and the temperature is typically such that the water vapor in the reaction exists as steam. In terms of pure stochiometry, the shift reaction requires at least a steam-to-CO ratio of 1 to proceed, however the equilibrium for H2 production is favored by high moisture content. Additionally, higher ratios may be utilized to avoid carbon deposition at a catalyst surface and formation of larger hydrocarbon molecules, among other operational reasons. Adequate performance typically requires that steam be present significantly in excess of the minimum stiochiometric ratio, and ratios equal or higher than two are frequently utilized.
The water-gas shift reaction is commonly utilized following gasification, where a fuel is converted into gaseous components by applying heat under pressure and chemically decomposing the fuel to produce synthesis gas (syngas) comprised of H2, CO, CO2, and other gaseous constituents. Following cooldown from gasification temperatures, the syngas may then be processed in a water-gas-shift reactor, where the CO and steam react to produce additional CO2 and increase the H2 concentration. Steam may be present in the syngas stream and may act to supply a portion of the steam-to-CO ratio required in the water-gas shift reactor, however typically an additional steam source is required in order to meet a steam-to-CO ratio that is operationally desired. Often the steam is generated during the syngas cooldown from the temperature condition of the gasifier to the desired temperature condition in the water-gas shift reactor, or it may be supplied directly from an alternate source. In terms of overall efficiency, this additional steam requirement is a parasitic load with a negative impact. Regardless of the origin, the requirement for additional steam consumes thermal energy which could be utilized to supply other steam loads.
Techniques exist to mitigate the requirement for additional steam sources. For example, water slurry feed gasification systems generate a more humid syngas, and often additional steam is not required for WGS reactor operation. However, a water slurry feed system results in thermal efficiency losses, and this problem is especially exacerbated with low ranked coals that already have a high water content. Another technique utilizes a dry feed gasifier to avoid the thermal efficiency losses, and injects water droplets into the relatively dry syngas in order to provide syngas cooldown and humidify the syngas stream. This humidification reduces the quantity of steam injection subsequently required for WGS reactor operation, and can somewhat mitigate the negative impact on plant efficiency. However, the careful balancing of the required thermal transfer to the injected water droplets for syngas cooldown and the resulting steam-to-CO ratio of the cooled syngas subsequently sent downstream may still require additional steam injection before encountering the WGS reactor. See e.g. Martelli et al, “Comparison of coal IGCC with and without CO2 capture and storage: Shell gasification with standard vs. partial water quench”, Energy Procedia 1 (2009).
It would provide a significant advantage to provide a process whereby a syngas stream comprised of CO, CO2, and H2 could be humidified prior to entering a WGS reactor, in order to reduce or eliminate parasitic steam injection into the syngas stream.
There has also been significant effort toward removing the CO2 present in syngas streams prior to WGS reactor entry in order to enhance H2 production. The water-gas shift is a reversible, equilibrium-limited reaction, and thus becomes hindered when the concentration of CO2 in the stream increases. As is well understood, if the concentration of CO2 in the syngas stream is reduced prior to entry into the WGS reactor, the equilibrium of the water-gas shift reaction is shifted in favor of the forward reaction products, and conversion of CO and H2O to CO2 and H2 increases in the WGS reactor. One practiced approach is the use of a CO2-selective membrane on the syngas stream in order to remove some portion of the present CO2 prior to WGS reactor entry. See U.S. Pat. No. 7,011,694 to Ho, issued Mar. 14, 2006, among others. Similarly, water-gas shift membrane reactors (WGS-MR) utilize membranes to remove either H2 or CO2 as they generate in-situ in the reactor, shifting the equilibrium toward greater conversion to H2. See U.S. Pat. No. 6,090,312 to Ziaka et al, issued Jul. 18, 2000, among others. These approaches effectively act to remove CO2 or H2 and shift the equilibrium favorably, however preparing thin and durable membranes is a challenge, and higher temperature operation can be difficult. An alternate approach utilizes a CO2 sorbent such as lime or dolomite for CO2 removal. See U.S. Pat. No. 7,354,562 to Ying et al, issued Apr. 8, 2008, among others. Like the membrane approaches, CO2 removal is realized, however these methods do not act to mitigate any additional steam injection into the syngas stream that may be required.
It would provide a significant advantage to provide a process whereby CO2 removal from a syngas stream comprised of CO, CO2, and H2 could be accomplished in a manner that further humidifies the syngas prior to entering a WGS reactor, in order to shift the water-gas shift reaction favorably while simultaneously reducing or eliminating steam injection requirements into the syngas stream.
There are similarly gasification processes which report the removal of CO2 and the production of an enriched H2 product through the use of CO2 sorbents within a coal gasification reactor. For example, calcium hydroxide (Ca(OH)2) has been used for the sorption of CO2 with liberation of H2O in coal gasification products, in order to absorb CO2 as it originates during gasification and provide an H2 enriched product. In these processes the molecular dispersion of the organic and oxidant reactants is conducive to rapid oxidation reactions and high H2 production, and the use of a separate water-gas-shift step can be avoided. However, the process is most successful under supercritical conditions, which imposes severe operational requirements. Under subcritical conditions, the process tends to produce a higher level of methane, which would likely necessitate WGS reactor operations following methane reforming in order to optimize the production of H2. See Kuramoto et al, “Coal gasification with subcritical steam in the presence of a CO2 sorbent: products and conversion under transient heating,” Fuel Processing Technology 82 (2003). In the subcritical operation, a methodology whereby a sorbent removes CO2 and humidifies the syngas stream prior to entry into the WGS reactor would continue to remain valuable.
In an integrated plant such as an IGCC which utilizes a WGS reactor to boost the hydrogen concentration prior to hydrogen combustion, carbon capture strategies also impose negative impacts on overall plant efficiency, as is well known. The impacts are worsened when the carbon capture strategy requires absorption of CO2 at low temperature and desorption at low pressure, and/or utilizes a solvent sensitive to water content in the synthesis gas stream, necessitating water removal operations. These impacts exist in both pre-combustion and post-combustion carbon capture strategies, although pre-combustion capture in an IGCC plant offers distinct advantages, because the CO2 is relatively concentrated in the WGS reactor exit stream and the driving force for various types of separation and capture technologies is significantly improved. As a result, there are significant efficiency advantages to using a CO2 capture sorbent operable at WGS reactor temperatures for pre-combustion capture in an IGCC plant. It would be further advantageous if the sorbent material utilized for CO2 absorption and humidification of the syngas stream prior to entry into the WGS reactor was additionally utilized for CO2 capture following exit from the WGS reactor, so that regeneration and CO2 separation for both processes could occur under similar conditions, and so that the regeneration processes could be combined in order to mitigate energy and resource requirements arising from cyclic use of the sorbent material for both before and after a water-gas shift reaction.
Accordingly, it is an object of this disclosure to provide a process whereby a gaseous stream comprised of CO and CO2 can be humidified prior to entering a WGS reactor, in order to reduce or eliminate parasitic steam injection into the gaseous stream.
Further, it is an object of this disclosure to provide a process whereby CO2 removal from a gaseous stream comprised of CO and CO2 can be accomplished during the humidification process, in order to shift the water-gas shift reaction toward the favorable forward reaction products.
Further, it is an object of this disclosure to utilize a sorbent material for both CO2 absorption and humidification of the gaseous stream prior to entry into the WGS reactor, and to further utilize the sorbent material for CO2 capture following exit from the WGS reactor, in order to produce an enriched H2 stream.
Further, it is an object of this disclosure to conduct CO2 absorption, gaseous stream humidification, and CO2 capture following the WGS reactor using a sorbent regenerable to approximate WGS reactor temperature and pressure conditions, in order to mitigate energy penalties associated with CO2 capture and sequestration.
Further, it is an object of this disclosure to conduct CO2 absorption, gaseous stream humidification, and CO2 capture using a sorbent which tolerates H2O in the gaseous stream, in order to mitigate water removal requirements and allow operation of H2O containing streams, such as a syngas stream comprised H2, CO, CO2, and H2O.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.